Watercolour Paint Mediums & Varnishes

It is possible to varnish a watercolour painting, but it is important to note that the action is irreversible. Thus, this process is not recommended for beginners. Application of the varnish will also alter the texture, colour, and general feel of the watercolour painting, so keep that in mind before applying to make sure the result is desired. Naturally, additional protection will be present after the varnish. 

Yes, glazing with watercolours is often referred to an action, called laying down a wash. A layer of transparent colour is washed over a previous dry layer of paint. An individual can then see both layers (or more) of paint and their eye visually mixes the colours. This technique makes the colours more luminous than when simply mixed on the palette.